1. Field of the Invention
This invention relates to an oxygen concentration detector for detecting the oxygen concentration of a gas, which detector is particularly suitable for detecting the oxygen concentration of a gas at a low temperature.
2. Description of the Prior Art
To detect the oxygen concentration in exhaust gas from internal combustion engine, oxygen concentration detectors having metal oxide semiconductors, such as titania (TiO.sub.2) or zinc oxide (ZnO), whose electric resistance varies with the oxygen partial pressure in the ambient gas have been known.
The oxygen concentration detectors of the prior art using the above-mentioned metal oxide semiconductors have shortcomings in that the resistance value thereof at low temperatures is very high for both the deficient oxygen concentration and the excess oxygen concentration, and such very high resistance value renders the oxygen concentration detector susceptible to adverse effects of noises, and in that a large time lag exists in variation of the resistance value in response to a change in the oxygen concentration of the exhaust gas. Due to such shortcomings, the lowest temperature limit for practical use of the above-mentioned conventional oxygen concentration detectors has been about 300.degree. C. On the other hand, the temperature of the exhaust gas from internal combustion engines is below the above-mentioned lowest temperature limit at the start or during slow running of the engines, so that the conventional oxygen concentration detectors with metal oxide semiconductors have not been used to their full capability.
To solve such shortcomings, an oxygen sensor with a heater disposed in juxtaposition with a metal oxide semiconductor has been proposed. However, the proposed use of the heater has a shortcoming in that, since the temperature of the gas to be measured such as the exhaust gas of internal combustion engines can be very high, the heater material must withstand the high temperature without any deterioration and noble metals such as platinum and rhodium are often used, and due to the low specific resistance of the noble metals and other heat-resistive materials the heater wires must be very thin, so that the structure of the heater becomes complicated and large and the heater wires are susceptible to breakage. Besides, a large power of several tens of watts is necessary to heat the metal oxide semiconductor.
Furthermore, the resistance value of the metal oxide semiconductor varies greatly with temperature, so that temperature correction of the resistance value has been necessary to determine the deficient air (rich burn) or excess air (lean burn).
In view of the fact that the oxygen concentration of the exhaust gas varies noticeably when the air-fuel ratio .lambda. thereof is in the proximity of 1.0, the oxygen concentration detectors of the above-mentioned type have been used mostly for controlling the air-fuel ratio .lambda. of the exhaust gas of internal combustion engine at 1.0 by utilizing the large change of resistance value of the metal oxide semiconductor in response to the large variation of the oxygen concentration thereat. However, with the conventional oxygen concentration detectors, it has been impossible to provide the rich burn control (.lambda.&lt;1.0) for ensuring a high output efficiency or the lean burn control (.lambda.&gt;1.0) for ensuring a good fuel efficiency.